Clean water is important for many industrial processes and for our daily life. The petroleum industry generates large amounts of wastewaters with high concentrations of oil, including produced water brought to the surface during oil drilling and gas production and refinery wastewater. Produced water accounts for the largest portion of wastewaters in petroleum industry and contains a wide range of contaminants, including salts, heavy metals, oil, suspended solid particles, dissolved organics, and small amount of chemical additives used for drilling, and its composition varies from well to well and from time to time. Depending on its use, produced water needs to be treated at different levels to ensure its reuse and recycling within oil and gas drilling operations, beneficial reuse outside of operations, and surface discharge. However, no matter for onshore disposal or reuse as process water or for off-shore discharge into the sea, essentially almost all oil and grease contaminants in produced water must be removed. Refinery wastewater, which constitutes another large stream of wastewaters, contains hydrocarbons even after conventional wastewater treatment due to its limited biological degradation, and thus also needs further treatment to remove remaining hydrocarbons for discharge or reuse. Hydrocyclones and dissolved air flotation have been used to quickly and effectively remove a large portion of the free oil droplets, but the quality of thus treated water is not high enough for discharge or reuse. Membrane filtration is a highly promising technology to further treat the resultant water with low concentration of oil to obtain almost oil-free water.
Membrane technologies (e.g., ultrafiltration and nanofiltration) widely used in water purification due to their high energy efficiency, low-cost, and simplicity for operation and maintaining. Ultrafiltration membranes with pore sizes from several to 100 nm are appropriate for oil/water separation and have been used to remove essentially all the emulsified oil from oil/water res. Surface property of the membrane materials also plays important role of determining oil/water separation performance. Various materials, such as ceramics (Al2O3, ZrO2, TiO2, SiO2, etc.), polymer, and carbon nanotubes, have been used to prepare ultrafiltration membranes for oil/water separation; depending on the materials hydrophobicity and hydrophilicity, either oil or water can be extracted from oil/water mixtures. However, it is more practical to use hydrophilic ultrafiltration membranes to selectively extract water from oil to avoid serious membrane fouling problems, as discussed below, because oils are heavier, more viscous and thus are expected to cause fouling quickly.
An ideal membrane for oil/water separation would have i) high water flux, ii) high rejection for oil, iii) long-term stability of flux and rejection, iv) excellent mechanical, chemical and thermal stability, v) ease of processing into large-scale membranes and modules, and vi) low-cost. Currently, long-term membrane stability or membrane fouling is a serious problem for many commercial ultrafiltration membranes and inhibits their wide application for oil/water separation; they usually experience substantial irreversible flux decrease when exposed to oil/water emulsion. Two types of membrane fouling usually occur in ultrafiltration: surface and internal. Surface fouling results from the deposition of contaminants on the membrane surface, and high speed surface flushing may remove surface contaminants and recover membrane performance. Internal fouling is caused by penetration of particulates into the membrane interior and thus pore blocking; penetrated particulates are difficult to be removed and typically lead to permanent membrane performance loss. With current ultrafiltration membrane materials and configuration (three dimensional pore structures and relatively thick membrane thickness), membrane fouling seems difficult to be solved.
Mixture separation using membrane technology can greatly reduce energy cost in industrial processes. The core of this technology is the highly selective membranes with high flux. Graphene, a single layer of graphite, has been considered as an ideal membrane material because it is extremely thin and thus can provide high permeation flux. However, currently there are no technologies available to fabricate ultrathin (<5 nm), graphene-based membranes that can highly selectively separate mixtures.